1. Field of the Invention
This invention relates to an improved fuel injection system for injecting fuel into internal combustion engines.
2. Description of the Prior Art
Stroke-controlled injection systems with a high-pressure reservoir (common rail) for introducing fuel into direct-injection internal combustion engines are known. The advantage of these injection systems is that the injection pressure can be adapted over wide ranges to the load and rpm. To reduce emissions and to attain high specific output, a high injection pressure is necessary. The attainable pressure level of high-pressure fuel pumps is limited for reasons of strength, so that to further increase the pressure in fuel injection systems, pressure boosters are used in the fuel injectors.
For introducing fuel into direct-injection internal combustion engines, stroke-controlled injection systems with a high-pressure reservoir (common rail) are used. The advantage of these injection systems is that the injection pressure can be adapted over wide ranges to the load and rpm. To reduce emissions and to attain high specific output, a high injection pressure is necessary. The attainable pressure level of high-pressure fuel pumps is limited for reasons of strength, so that to further increase the pressure in fuel injection systems, pressure boosters are used in the fuel injectors.
German Patent Disclosure DE 101 23 913 discloses a fuel injection system for internal combustion engines, with a fuel injector that can be supplied from a high-pressure fuel source. Connected between the fuel injector and the high-pressure fuel source is a pressure booster device that has a movable pressure booster piston. The pressure booster piston divides a chamber that can be connected to the high-pressure fuel source from a high-pressure chamber that communicates with the fuel injector. By filling a differential pressure chamber of the pressure booster device with fuel, or evacuating the differential pressure chamber of fuel, the fuel pressure in the high-pressure chamber can be varied. The fuel injector has a movable closing piston for opening and closing injection openings. The closing piston protrudes into a closing pressure chamber, so that the closing piston can be subjected to fuel pressure to attain a force acting in the closing direction. The closing pressure chamber and the differential pressure chamber are formed by a common closing pressure differential pressure chamber; all the subsidiary regions in the closing pressure differential pressure chamber communicate with one another permanently for exchanging fuel. A pressure chamber is provided for supplying the injection openings with fuel and subjecting the closing piston to a force acting in the opening direction. A high-pressure chamber communicates with the high-pressure fuel source in such a way that in the high-pressure chamber, aside from pressure fluctuations, at least the fuel pressure of the high-pressure fuel source can always be applied; the pressure chamber and the high-pressure chamber are formed by a common injection chamber. All the subsidiary regions of the injection chamber communicate permanently with one another for exchanging fuel.
German Patent Disclosure DE 102 294 15.1 relates to a device for needle stroke damping in pressure-controlled fuel injectors. A device for injecting fuel into a combustion chamber of an internal combustion engine is disclosed that includes a fuel injector which can be subjected to fuel that is at high pressure via a high-pressure source. The fuel injector is actuated via a metering valve, and an injection valve member is surrounded by a pressure chamber, and the injection valve member can be urged in the closing direction by a closing force. The injection valve member is assigned a damping element, which is movable independently of it and which defines a damping chamber and has at least one overflow conduit for connecting the damping chamber to a further hydraulic chamber. In DE 102 294 15.1, the control of the fuel injector is effected with a 3/2-way valve, and as a result, although an injector that is economical in both cost and installation space can be defined, nevertheless this valve must control a relatively large return quantity of the pressure booster.
Instead of the embodiment of a 3/2-way valve known from DE 102 294 15.1, servo valves may also be used, which in the state of repose of the servo valve are embodied in nonleaking fashion on the guide portion, which is favorable to the efficiency of a fuel injector. A disadvantage, however, is the fact that in the opened state of the servo valve piston of the 3/2-way valve, no pressure face pointing in the opening direction of the piston is subjected to system pressure. As a result, the movement of the servo valve piston in its housing is quite vulnerable to production tolerances. Moreover, a slow opening speed of the servo valve piston cannot be attained, and thus the minimum-quantity capacity of a servo valve configured in this way is limited. In the opened state of the servo valve piston, only an inadequate closing force ensues at a second valve seat embodied on it, and the result can be leaks and increased wear.